I have a 5v relay set up in this configuration http://arduino.sundh.com/wp-content/uploads/2011/10/transistor_TIP120_12V_g5LE-1.png The PIR sensor is working fine until the ground is joined (AR to relay breadboard). Then, the LED remains on and does not trip the relay. Oddly enough, this isn't always the case. Infrequently, it will work fine for a few minutes, then fail, regardless of the board (Uno, Pro Mini). Here is the sketch.

if(digitalRead(pirPin) == LOW){ digitalWrite(ledPin, LOW); //the led visualizes the sensors output pin state digitalWrite(relayPin,HIGH); if(takeLowTime){ lowIn = millis(); //save the time of the transition from high to LOW takeLowTime = false; //make sure this is only done at the start of a LOW phase } //if the sensor is low for more than the given pause, //we assume that no more motion is going to happen if(!lockLow && millis() - lowIn > pause){ //makes sure this block of code is only executed again after //a new motion sequence has been detected lockLow = true; Serial.print("motion ended at "); //output Serial.print((millis() - pause)/1000); Serial.println(" sec"); delay(50); } }}

Thank you so much! I have been beating this monkey for days. The inexperienced can find great web resources most of the time but can't often distinguish good from bad. Also, the ABC link will be very helpful.

If you have a 5V relay it depends how much current it needs.A small relay (say 5V 100mA) could perhaps be powered by the 5V pin of the Arduino.Using a darlington transistor at 5V is not so good, because of the voltage drop. But I think it will work.

Hi,do not trigger the relay from an arduino pin directly: you will almost surely destroy the pin and probably the arduino.The easiest way is: use a "logic level mosfet" like the 2N2007. You can drive it directly from a digital pin (no resistor needed): that is the gate is connected to the digital pin, the source to ground and the drain to the load. Do no forget to put a "kickback" diode (google that I dont remember the details) to protect the mosfet from the high voltage around the relay coil when switching it off.Check the power requirement as the 2N2007 is not really powerful.HTH